In a mobile communication system, there has recently been suggested a system of a base station called femtocell which covers a small area in comparison to that of a conventional base station (wide area base station, macrocell). The femtocell is a small base station installed in a house, a small office and the like, for example. While a cell of a conventional mobile phone, that is, an area covered by the wide area base station is about 1 to several kilometers in radius, an area covered by the femtocell is only about a few to tens of meters.
The femtocell is connected to the mobile communication network via the public link (broadband link such as ADSL or the like) drawn into each house. The public link has been in widespread use as an access link. Hence, if the broadband link is available, a user even outside the service area of the macrocell can easily use mobile phone services (a telephone call, a message function, an SMS (Short Message Service), a WEB browsing function and the like) similar to those provided by the macrocell, at a low cost. In addition, there is an advantage also for a carrier that it can improve a mobile phone area at a low cost, as usage of the femtocell and the broadband link by a user enables the carrier to save a resource (band or the like) of an existing wide area base station, which is originally supposed to be consumed.
Therefore, it is expected that, in 3 Generation (3G), 3.9 Generation (3.9G) and IMT-ADVANCED using a frequency band of 2 GHz or more, it becomes essential to combine the wide area base station placed outdoors and the femtocell installed indoors. In addition, since it is considered preferable that only registered terminals (mobile terminals) are allowed to use the femtocell, a limited number of users registered can occupy the femtocell in such a preferred condition. Accordingly, the femtocell has an advantage to offer a faster data communication environment of better quality in comparison to the macrocell which may be overcrowded with multiple users, and thus it is expected to be popularly used in the future.
Because of the advantages of the femtocell described above, it may be likely that the user capable of using a femtocell service wishes to connect to the femtocell when entering a femtocell service area from a macrocell service area. As a method to perform handoff from the macrocell to the femtocell, it is assumed to use a pilot beacon. Although it is desired that the femtocell uses a frequency different from that used by the macrocell in order to avoid interference with the macrocell, it is preferred to transmit the pilot beacon at the same frequency as that of the macrocell in order to lead the terminal communicating with the macrocell to the femtocell. However, in this case, there is a problem that the pilot beacon and the macrocell interfere with each other.
In order to handle such a problem, there is suggested a conventional scheme to prevent an adverse effect on other systems using the same frequency (for example, see Patent Document 1). According to the scheme described in Patent Document 1, transmission of a beacon frame is stopped if there is no wireless station terminal in a wireless area of an access point of a wireless LAN.